The demand for increased bandwidth in fiber optic telecommunications has driven the development of sophisticated transmitter lasers usable for dense wavelength division multiplexing (DWDM) systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. The International Telecommunications Union (ITU) presently requires channel separations of 50 GHz, or approximately 0.4 nanometers, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.
DWDM systems have largely been based on distributed feedback (DFB) lasers where spatial variations of the waveguide structure allow for a single wavelength operation Due to lack of control over the wavelength individual DFB transmitters are usable only for a single channel or a small number of adjacent channels.
Tunable external cavity lasers with internal frequency lockers or etalons have been developed to overcome the limitations of individual DFB devices. One drawback common to many external cavity lasers with intra-cavity etalons is an asymmetric mode pulling effect during laser operation that hinders stable, single mode lasing and complicates servo functions associated with laser operation. There is accordingly a need for external cavity laser and tuning mechanism that provides stable, single mode lasing. The present invention satisfies this need, as well as others, and overcomes the deficiencies found in the background art.